From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 879ABC76191 for ; Thu, 25 Jul 2019 18:26:08 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 65CF7216C8 for ; Thu, 25 Jul 2019 18:26:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726583AbfGYS0F convert rfc822-to-8bit (ORCPT ); Thu, 25 Jul 2019 14:26:05 -0400 Received: from mx1.redhat.com ([209.132.183.28]:63389 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725800AbfGYS0F (ORCPT ); Thu, 25 Jul 2019 14:26:05 -0400 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 2C26D30001D8; Thu, 25 Jul 2019 18:26:04 +0000 (UTC) Received: from [10.18.17.163] (dhcp-17-163.bos.redhat.com [10.18.17.163]) by smtp.corp.redhat.com (Postfix) with ESMTPS id CD0165E1B0; Thu, 25 Jul 2019 18:25:53 +0000 (UTC) Subject: Re: [PATCH v2 QEMU] virtio-balloon: Provide a interface for "bubble hinting" To: Alexander Duyck , "Michael S. Tsirkin" Cc: Alexander Duyck , kvm@vger.kernel.org, david@redhat.com, dave.hansen@intel.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, akpm@linux-foundation.org, yang.zhang.wz@gmail.com, pagupta@redhat.com, riel@surriel.com, konrad.wilk@oracle.com, lcapitulino@redhat.com, wei.w.wang@intel.com, aarcange@redhat.com, pbonzini@redhat.com, dan.j.williams@intel.com References: <20190724165158.6685.87228.stgit@localhost.localdomain> <20190724171050.7888.62199.stgit@localhost.localdomain> <20190724173403-mutt-send-email-mst@kernel.org> <20190725111303-mutt-send-email-mst@kernel.org> <96b1ac42dccbfbb5dd17210e6767ca2544558390.camel@linux.intel.com> From: Nitesh Narayan Lal Openpgp: preference=signencrypt Autocrypt: addr=nitesh@redhat.com; prefer-encrypt=mutual; keydata= mQINBFl4pQoBEADT/nXR2JOfsCjDgYmE2qonSGjkM1g8S6p9UWD+bf7YEAYYYzZsLtbilFTe z4nL4AV6VJmC7dBIlTi3Mj2eymD/2dkKP6UXlliWkq67feVg1KG+4UIp89lFW7v5Y8Muw3Fm uQbFvxyhN8n3tmhRe+ScWsndSBDxYOZgkbCSIfNPdZrHcnOLfA7xMJZeRCjqUpwhIjxQdFA7 n0s0KZ2cHIsemtBM8b2WXSQG9CjqAJHVkDhrBWKThDRF7k80oiJdEQlTEiVhaEDURXq+2XmG jpCnvRQDb28EJSsQlNEAzwzHMeplddfB0vCg9fRk/kOBMDBtGsTvNT9OYUZD+7jaf0gvBvBB lbKmmMMX7uJB+ejY7bnw6ePNrVPErWyfHzR5WYrIFUtgoR3LigKnw5apzc7UIV9G8uiIcZEn C+QJCK43jgnkPcSmwVPztcrkbC84g1K5v2Dxh9amXKLBA1/i+CAY8JWMTepsFohIFMXNLj+B RJoOcR4HGYXZ6CAJa3Glu3mCmYqHTOKwezJTAvmsCLd3W7WxOGF8BbBjVaPjcZfavOvkin0u DaFvhAmrzN6lL0msY17JCZo046z8oAqkyvEflFbC0S1R/POzehKrzQ1RFRD3/YzzlhmIowkM BpTqNBeHEzQAlIhQuyu1ugmQtfsYYq6FPmWMRfFPes/4JUU/PQARAQABtCVOaXRlc2ggTmFy YXlhbiBMYWwgPG5pbGFsQHJlZGhhdC5jb20+iQI9BBMBCAAnBQJZeKUKAhsjBQkJZgGABQsJ CAcCBhUICQoLAgQWAgMBAh4BAheAAAoJEKOGQNwGMqM56lEP/A2KMs/pu0URcVk/kqVwcBhU SnvB8DP3lDWDnmVrAkFEOnPX7GTbactQ41wF/xwjwmEmTzLrMRZpkqz2y9mV0hWHjqoXbOCS 6RwK3ri5e2ThIPoGxFLt6TrMHgCRwm8YuOSJ97o+uohCTN8pmQ86KMUrDNwMqRkeTRW9wWIQ EdDqW44VwelnyPwcmWHBNNb1Kd8j3xKlHtnS45vc6WuoKxYRBTQOwI/5uFpDZtZ1a5kq9Ak/ MOPDDZpd84rqd+IvgMw5z4a5QlkvOTpScD21G3gjmtTEtyfahltyDK/5i8IaQC3YiXJCrqxE r7/4JMZeOYiKpE9iZMtS90t4wBgbVTqAGH1nE/ifZVAUcCtycD0f3egX9CHe45Ad4fsF3edQ ESa5tZAogiA4Hc/yQpnnf43a3aQ67XPOJXxS0Qptzu4vfF9h7kTKYWSrVesOU3QKYbjEAf95 NewF9FhAlYqYrwIwnuAZ8TdXVDYt7Z3z506//sf6zoRwYIDA8RDqFGRuPMXUsoUnf/KKPrtR ceLcSUP/JCNiYbf1/QtW8S6Ca/4qJFXQHp0knqJPGmwuFHsarSdpvZQ9qpxD3FnuPyo64S2N Dfq8TAeifNp2pAmPY2PAHQ3nOmKgMG8Gn5QiORvMUGzSz8Lo31LW58NdBKbh6bci5+t/HE0H pnyVf5xhNC/FuQINBFl4pQoBEACr+MgxWHUP76oNNYjRiNDhaIVtnPRqxiZ9v4H5FPxJy9UD Bqr54rifr1E+K+yYNPt/Po43vVL2cAyfyI/LVLlhiY4yH6T1n+Di/hSkkviCaf13gczuvgz4 KVYLwojU8+naJUsiCJw01MjO3pg9GQ+47HgsnRjCdNmmHiUQqksMIfd8k3reO9SUNlEmDDNB XuSzkHjE5y/R/6p8uXaVpiKPfHoULjNRWaFc3d2JGmxJpBdpYnajoz61m7XJlgwl/B5Ql/6B dHGaX3VHxOZsfRfugwYF9CkrPbyO5PK7yJ5vaiWre7aQ9bmCtXAomvF1q3/qRwZp77k6i9R3 tWfXjZDOQokw0u6d6DYJ0Vkfcwheg2i/Mf/epQl7Pf846G3PgSnyVK6cRwerBl5a68w7xqVU 4KgAh0DePjtDcbcXsKRT9D63cfyfrNE+ea4i0SVik6+N4nAj1HbzWHTk2KIxTsJXypibOKFX 2VykltxutR1sUfZBYMkfU4PogE7NjVEU7KtuCOSAkYzIWrZNEQrxYkxHLJsWruhSYNRsqVBy KvY6JAsq/i5yhVd5JKKU8wIOgSwC9P6mXYRgwPyfg15GZpnw+Fpey4bCDkT5fMOaCcS+vSU1 UaFmC4Ogzpe2BW2DOaPU5Ik99zUFNn6cRmOOXArrryjFlLT5oSOe4IposgWzdwARAQABiQIl BBgBCAAPBQJZeKUKAhsMBQkJZgGAAAoJEKOGQNwGMqM5ELoP/jj9d9gF1Al4+9bngUlYohYu 0sxyZo9IZ7Yb7cHuJzOMqfgoP4tydP4QCuyd9Q2OHHL5AL4VFNb8SvqAxxYSPuDJTI3JZwI7 d8JTPKwpulMSUaJE8ZH9n8A/+sdC3CAD4QafVBcCcbFe1jifHmQRdDrvHV9Es14QVAOTZhnJ vweENyHEIxkpLsyUUDuVypIo6y/Cws+EBCWt27BJi9GH/EOTB0wb+2ghCs/i3h8a+bi+bS7L FCCm/AxIqxRurh2UySn0P/2+2eZvneJ1/uTgfxnjeSlwQJ1BWzMAdAHQO1/lnbyZgEZEtUZJ x9d9ASekTtJjBMKJXAw7GbB2dAA/QmbA+Q+Xuamzm/1imigz6L6sOt2n/X/SSc33w8RJUyor SvAIoG/zU2Y76pKTgbpQqMDmkmNYFMLcAukpvC4ki3Sf086TdMgkjqtnpTkEElMSFJC8npXv 3QnGGOIfFug/qs8z03DLPBz9VYS26jiiN7QIJVpeeEdN/LKnaz5LO+h5kNAyj44qdF2T2AiF HxnZnxO5JNP5uISQH3FjxxGxJkdJ8jKzZV7aT37sC+Rp0o3KNc+GXTR+GSVq87Xfuhx0LRST NK9ZhT0+qkiN7npFLtNtbzwqaqceq3XhafmCiw8xrtzCnlB/C4SiBr/93Ip4kihXJ0EuHSLn VujM7c/b4pps Organization: Red Hat Inc, Message-ID: Date: Thu, 25 Jul 2019 14:25:52 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: <96b1ac42dccbfbb5dd17210e6767ca2544558390.camel@linux.intel.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8BIT Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.46]); Thu, 25 Jul 2019 18:26:04 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 7/25/19 12:16 PM, Alexander Duyck wrote: > On Thu, 2019-07-25 at 11:16 -0400, Michael S. Tsirkin wrote: >> On Thu, Jul 25, 2019 at 08:05:30AM -0700, Alexander Duyck wrote: >>> On Thu, 2019-07-25 at 07:35 -0400, Nitesh Narayan Lal wrote: >>>> On 7/24/19 6:03 PM, Alexander Duyck wrote: >>>>> On Wed, 2019-07-24 at 17:38 -0400, Michael S. Tsirkin wrote: >>>>>> On Wed, Jul 24, 2019 at 10:12:10AM -0700, Alexander Duyck wrote: >>>>>>> From: Alexander Duyck >>>>>>> >>>>>>> Add support for what I am referring to as "bubble hinting". Basically the >>>>>>> idea is to function very similar to how the balloon works in that we >>>>>>> basically end up madvising the page as not being used. However we don't >>>>>>> really need to bother with any deflate type logic since the page will be >>>>>>> faulted back into the guest when it is read or written to. >>>>>>> >>>>>>> This is meant to be a simplification of the existing balloon interface >>>>>>> to use for providing hints to what memory needs to be freed. I am assuming >>>>>>> this is safe to do as the deflate logic does not actually appear to do very >>>>>>> much other than tracking what subpages have been released and which ones >>>>>>> haven't. >>>>>>> >>>>>>> Signed-off-by: Alexander Duyck >>>>>> BTW I wonder about migration here. When we migrate we lose all hints >>>>>> right? Well destination could be smarter, detect that page is full of >>>>>> 0s and just map a zero page. Then we don't need a hint as such - but I >>>>>> don't think it's done like that ATM. >>>>> I was wondering about that a bit myself. If you migrate with a balloon >>>>> active what currently happens with the pages in the balloon? Do you >>>>> actually migrate them, or do you ignore them and just assume a zero page? >>>>> I'm just reusing the ram_block_discard_range logic that was being used for >>>>> the balloon inflation so I would assume the behavior would be the same. >>>> I agree, however, I think it is worth investigating to see if enabling hinting >>>> adds some sort of overhead specifically in this kind of scenarios. What do you >>>> think? >>> I suspect that the hinting/reporting would probably improve migration >>> times based on the fact that from the sound of things it would just be >>> migrated as a zero page. >>> >>> I don't have a good setup for testing migration though and I am not that >>> familiar with trying to do a live migration. That is one of the reasons >>> why I didn't want to stray too far from the existing balloon code as that >>> has already been tested with migration so I would assume as long as I am >>> doing almost the exact same thing to hint the pages away it should behave >>> exactly the same. >>> >>>>>> I also wonder about interaction with deflate. ATM deflate will add >>>>>> pages to the free list, then balloon will come right back and report >>>>>> them as free. >>>>> I don't know how likely it is that somebody who is getting the free page >>>>> reporting is likely to want to also use the balloon to take up memory. >>>> I think it is possible. There are two possibilities: >>>> 1. User has a workload running, which is allocating and freeing the pages and at >>>> the same time, user deflates. >>>> If these new pages get used by this workload, we don't have to worry as you are >>>> already handling that by not hinting the free pages immediately. >>>> 2. Guest is idle and the user adds up some memory, for this situation what you >>>> have explained below does seems reasonable. >>> Us hinting on pages that are freed up via deflate wouldn't be too big of a >>> deal. I would think that is something we could look at addressing as more >>> of a follow-on if we ever needed to since it would just add more >>> complexity. >>> >>> Really what I would like to see is the balloon itself get updated first to >>> perhaps work with variable sized pages first so that we could then have >>> pages come directly out of the balloon and go back into the freelist as >>> hinted, or visa-versa where hinted pages could be pulled directly into the >>> balloon without needing to notify the host. >> Right, I agree. At this point the main thing I worry about is that >> the interfaces only support one reporter, since a page flag is used. >> So if we ever rewrite existing hinting to use the new mm >> infrastructure then we can't e.g. enable both types of hinting. > Does it make sense to have multiple types of hinting active at the same > time though? That kind of seems wasteful to me. I agree. > Ideally we should be able > to provide the hints and have them feed whatever is supposed to be using > them. So for example I could probably look at also clearing the bitmaps > when migration is in process. > > Also, I am wonder if the free page hints would be redundant with the form > of page hinting/reporting that I have since we should be migrating a much > smaller footprint anyway if the pages have been madvised away before we > even start the migration. > >> FWIW Nitesh's RFC does not have this limitation. > Yes, but there are also limitations to his approach. For example the fact > that the bitmap it maintains is back to being a hint rather then being > very exact. True. > As a result you could end up walking the bitmap for a while > clearing bits without ever finding a free page. Are referring to the overhead which will be introduced due to bitmap scanning on very large guests? > >> I intend to think about this over the weekend. > Sounds good. I'll try to get the stuff you have pointed out so far > addressed and hopefully have v3 ready to go next week. > > Thanks. > > - Alex > -- Thanks Nitesh